Electrochemical displays can be arranged to form predetermined graphic images or to reveal previously formed images that have been blocked from sight. Sometimes, the rate of image formation or revelation is controlled to provide a timing or other measuring function. The graphic images can take a variety of forms from a patch of color to text, patterns, or pictures.
The displays generally include two electrodes, an electrolyte, and an electronic conductor between the two electrodes. The electrolyte provides an ionically conductive pathway between the electrodes, and the conductor provides an electronically conductive pathway between the electrodes. Voltaic display cells have electrodes with different electrode potentials. Completing both the pathways (ionic and electronic) between the different electrodes triggers an electrochemical reaction that produces visible changes in the displays. Electrolytic display cells have electrodes with equal electrode potentials, but similar electrochemical reactions can be started by imposing electrical potentials across the two electrodes from external power sources.
Activation of electrolytic display cells can be controlled by single or double pole switches that interrupt the flow of current to the electrodes or that change its direction. For example, U.S. Pat. No. 4,153,345 to Duchene et al. discloses an electrolytic display cell in which a pattern of thin metal film is alternately dissolved into a liquid electrolyte and redeposited onto conductive portions of a transparent electrode. The polarity of an external power source controls whether the display undergoes dissolution or deposition. Electrolytic cells can also be connected to external power sources for monitoring the condition of external sources. For example, U.S. Pat. No. 5,418,086 to Bailey discloses an electrolytic-type battery charge indicator powered by the monitored battery. One electrode is dissolved and redeposited onto another electrode as an indication of battery usage.
Activation of voltaic cells can be controlled by interrupting either the ionically conductive pathway or the electronic conductive pathway between electrodes. For example, U.S. Pat. No. 4,894,275 to Kang et al. discloses a self-powered electrochromic timing device in which a color change boundary in an electrochromic material is advanced by a gradual dissolution of an electrode in the presence of an electrolyte. Activation is deferred by isolating or deactivating the electrolyte or by physically separating portions of the electrodes intended for electronic connection. U.S. Pat. No. 5,339,024 to Kuo et al. discloses a voltaic-type charge indicator cell connected in parallel with a main cell. One electrode is dissolved by an amount related to battery condition.